Power converter suitable for high frequencies

ABSTRACT

A switching circuit comprises a main switch element having a gate as a control input; and a ring oscillator connected as a driver circuit to the gate to drive the main switch via the gate. The basic circuit is used to build various components which have the property that they can work at very high frequencies.

RELATED APPLICATION SECTION

This application claims the benefit of priority of U.S. ProvisionalApplication No. 62/881,436 filed 1 Aug. 2019, the contents of which areincorporated herein by reference in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to a powerconverter that is suitable for high frequencies and, more particularly,but not exclusively, to such a power converter that may be built into anintegrated circuit.

Power converters rely on externally applied periodic signals to transferenergy from a source to a load through inductors, capacitors andswitches. The limitation of externally controlled power converters isthat the external periodic signals require critical design and complexdesign issues. Furthermore, the frequency of operation is limited by theefficiency of the drive transistors and diodes (including, generally,synchronous rectifiers).

Self-oscillating power converters exist that use one or two transistorsin a simple multivibrator circuit or in a fly-back configuration.Generally, the power efficiency is limited by the transistor powerdissipation.

Existing converters, particularly for high frequencies, are relativelylarge in size and their efficiency gets poorer as the frequency rises.Yet, a high frequency converter that is small enough to be built into anintegrated circuit is highly desirable for fields as diverse asautonomous cars, space equipment and others.

Electric energy converters, which can effectively interface energystorage devices such as batteries, supercapacitors and reversible fuelcells to power conversion systems within applications of electricalvehicles (EVs), renewable energy generation (REG), uninterruptible powersupplies (USPs) etc., have gained increasing attention in academia andindustry over the last decade. Moreover, with the emergence of applyingwide bandgap devices such as Silicon Carbide (SiC) and Gallium Nitride(GaN) based power switches, power electronic converters tend to be evenfaster, smaller and more efficient, due to the increased electricalfield strength and electron mobility compared to silicon (Si) basedcounterparts. A high switching frequency, to some extent, can certainlyoffer opportunities for converter volume reduction and thereby obtainhigher power density and more compact design. However, it is alsoaccompanied by new challenges, for instance, increased switching losseseven with wide bandgap devices, worse electromagnetic interference (EMI)and more stress on magnetic components. Therefore, soft-switchingtechnologies, including zero-voltage switching and zero-currentswitching, are still widely used in the field of applications of wideband-gap semiconductors. High-frequency soft-switching GaN-basedimplementations under boundary conduction mode have been reported in theliterature, in which the reduction of switching loss can well compensatefor the increased conduction loss. For instance, a 5-MHz boost converterwith efficiency up to 98% was demonstrated. A boost converter is aconverter that converts a lower voltage into a higher voltage. Anon-inverting Buck-Boost converter, as shown was proposed, and it isable to achieve full good operation, as well as flexible and easycontrol, which is significantly better than resonant converters such asClass-E DC-DC converters. More recently a high switching frequencyGaN-based bidirectional DC-DC converter has been presented, which mayachieve relatively high efficiency and high power density by carefuldesign, in particular selecting an optimum dead time. The convertershows that the conduction loss increases accordingly when the dead timeis longer due to the larger forward voltage drop of GaN devices, andthus a problem arises in that setting up ideal switching conditions forGaN devices is more difficult than for their silicon-based counterparts.

A similar issue arises with class-D amplifiers. Class D, or switching,amplifiers are those in which the amplifying devices act as switches.The existing class-D amplifiers today are relatively large in size andtheir efficiency is poor. These characteristics are particularly crucialwhen applied in autonomous cars, space equipment and others.

SUMMARY OF THE INVENTION

The present embodiments may overcome the problem of switching dead timeby using a three phase or three stage ring oscillator as the basis forthe converter. The ring oscillator may inherently provide a moreeffective power conversion due to precise timing within the ringoscillator stages.

The present embodiments may implement a converter using GaN technologyin a ring oscillator configuration. The ring oscillator circuit maygenerate its own conversion frequency which may be much higher thancontrolled circuitry. The ring oscillator may be provided in multiplestages, for example two stages, three stages, five stages etc.Increasing the number of stages or phases decreases the ripple andallows for higher voltages. Arranging multiple stages in a sequence ofsteps allows for an increase in current and thus power as well.

In the case of a class D amplifier, embodiments may provide an odd-phaseswitching (Class-D) power amplifier based on GaN devices and ringoscillator circuitry. The amplifier may be efficient to very highfrequencies, even RF, with high efficiency and small size compared tothe existing systems.

According to an aspect of some embodiments of the present inventionthere is provided a power converter comprising an input, a power outputand a plurality of ring oscillator stages connected between said inputand said power output, the ring oscillator stages each comprising atleast one enhancement mode transistor.

In an embodiment, said ring oscillator stages respectively comprise asecond enhancement mode transistor.

In an embodiment, the at least one enhancement mode transistor is atleast one member of the group consisting of a SiC transistor and aGallium Nitride transistor.

In an embodiment, the enhancement mode transistor is a wide bandgaptransistor.

In an embodiment, a drive connection of a first stage is connected viaan impedance to a following stage.

In an embodiment, at least one of said ring oscillator stages comprisesa pull-up transistor and a pull-down transistor.

In an embodiment, said drive connection is between said pull-uptransistor and said pull-down transistor.

In an embodiment, said impedance comprises an inductor.

In an embodiment, a drive connection of a final oscillator stage isconnected via an impedance to a first ring oscillator stage.

In an embodiment, each ring oscillator stage is connected via an inputimpedance to said input.

In an embodiment, said input impedance comprises an inductance.

Embodiments may include a cascade of at least three of said ringoscillator stages.

According to a further aspect of the present invention there is provideda power converter comprising an input, a power output and a plurality ofring oscillator stages connecting said input to said output, wherein adrive connection of a first stage is connected via an impedance to afollowing stage.

In an embodiment, the impedance comprises at least an inductor.

In an embodiment, said ring oscillators respectively comprise at leasttwo enhancement mode transistors.

In an embodiment, a drive connection of a final oscillator stage isconnected via an impedance to a first ring oscillator stage.

The power converter may include three of said ring oscillators, each ofsaid three ring oscillators driving a switch for a different phase of athree phase system.

According to a yet further aspect of the present invention there isprovided an amplifier comprising an input, a power output and aplurality of ring oscillator stages connected between said input andsaid power output, the ring oscillator stages each comprising at leastone enhancement mode transistor.

In an embodiment, a drive connection of a first ring oscillator stage isconnected via an impedance to a following ring oscillator stage.

A drive connection of a final oscillator stage of the amplifier may beconnected via an impedance to a first ring oscillator stage.

The amplifier may be a D-type switching amplifier.

A yet further aspect of the present invention may provide a switchingcircuit comprising: A main switch circuit having a gate as a controlinput; and A ring oscillator connected as a driver circuit to said gateto drive said main switch via said gate.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIG. 1 is a simplified diagram showing a ring oscillator according tothe existing art;

FIG. 2 is a theoretical diagram showing ring oscillator connected withenhancement mode transistors to drive an output according to a firstembodiment of the present invention;

FIG. 3 is a circuit diagram of a simplified circuit using ringoscillators according to embodiments of the present invention;

FIG. 4 is a graph showing voltages at two different locations in thesimplified circuit of FIG. 3;

FIG. 5 is a simplified circuit diagram of a circuit using three ringoscillators according to embodiments of the present invention;

FIG. 6 is a graph of the switching voltage of the circuit of FIG. 5;

FIG. 7 is a diagram of a single stage of a multi-stage circuit in whichan amplifier is isolated by pull up and pull down networks in a ringoscillator;

FIG. 8 is a three-ring-oscillator-stage circuit built using the units ofFIG. 7 to form a buck circuit;

FIG. 9 is a three-ring-oscillator-stage circuit forming a boost circuit;

FIG. 10 is a simplified graph showing voltage outputs from the circuitof FIG. 9;

FIG. 11 is a simplified diagram of a D-type amplifier constructed usingring oscillators according to embodiments of the present invention;

FIG. 12 shows the oscilloscope output for voltages at the three gates ofa three-ring oscillator stage amplifier according to embodiments of thepresent invention;

FIGS. 13A and 13B illustrate the underside of an integrated circuit withconnector pins and an indication of how an additional pin can be used toconnect between the different ring oscillator stages in an embodiment ofthe present invention;

FIG. 14 is a simplified circuit diagram of a three-ring-oscillator boostcircuit according to embodiments of the present invention; and

FIGS. 15 to 18 are three different five-ring-oscillator-stage circuitsaccording to embodiments of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to a highfrequency power converter and, more particularly, but not exclusively,to a power converter that may be built on an integrated circuit.

The present embodiments may provide a multiple phase, interleaved powerconverter based on enhancement-mode only devices. Implementations may beemployed in any electric storage system where high efficiency andminiature size are required and embodiments may achieve efficiencies of90% at frequencies of 100 MHz and may be built into individualintegrated circuits. The converter circuit consists of a ring oscillatorhaving at least 2 stages with an ideal phase shift of 360°/N that maytrigger the signal timing, and the converter circuit may be connected tothe power output terminals. The converter circuit is related to the RingOscillator test circuit described in US Patent Application Pub. No US2018/0246161 A1, the contents of which are hereby incorporated byreference herein in their entirety.

Ring oscillators are self-oscillating circuits whereby when sufficientvoltage is applied to a chain of inverters, they are inherently unstableand immediately begin oscillations with applied voltage. This phenomenonis generally used in CMOS circuits to determine the characteristicproperties of the transistors in use. The present embodiments addressthe problem of control by operating the devices at the natural ringfrequency of 2 or 3 stage enhancement-mode only transistors, thusallowing a power converter to be built on the basis of ring oscillators.Feedback from the output voltage allows the boost converter to controleither voltage or current at the output up to a maximum current orvoltage that the transistors will allow.

The present embodiments may use a wide bandgap transistor, such as ahigh electron mobility transistor (HEMT) and an exemplary candidate isan enhancement mode, normally off Gallium Nitride (GaN) device, whichmay operate at very high natural frequencies and very low switchinglosses. An HEMT, also known as a heterostructure FET (HFET) ormodulation-doped FET (MODFET), is a field-effect transistor whichincorporates a heterojunction as the channel. The heterojunctionreplaces the doped region that is used in a MOSFET. Indeed, in recentyears, gallium nitride HEMTs have attracted attention due to theirhigh-power performance.

While there are self-oscillating converters, these are limited to one ortwo transistors as mentioned, and certainly none of them use multiplestages or a ring-oscillator configuration. A configuration of threeequally phased ring oscillator stages may give a smooth power output ata maximum frequency determined by the speed of the switches, which forGaN-based transistors may be relatively high.

The current embodiments aim to provide an exemplary three or five phasepower converter based on GaN devices. SiC devices may alternatively beconsidered. The converter may be employed in any electric storagesystem, with high efficiency and small size compared to the existingsystems. The converter circuit may consist of multiple stages based on aring oscillator, for example three stages. The three stage embodimentmay have an ideal phase shift of 120° that may trigger signal timingwhich may be connected to the power output terminals.

In the case of a class-D amplifier, an odd-phase switching (Class-D)power amplifier based on GaN devices may be provided. The amplifier maybe efficient to very high frequencies, even RF, with high efficiency andsmall size compared to the existing systems. The amplifier circuit mayconsists of a ring oscillator having say three or more stages, with anideal phase shift of 360°/N that will trigger signal timing which may beconnected to the power output terminals. The amplifier input may be thegate voltage of a common node connected to the load transistors of eachphase (stage) so the input power will be very low compared to the powerdelivered at the output. The construction according to the presentembodiments may guarantee high gain and high efficiency even atfrequencies approaching, and even exceeding, GHz.

For purposes of better understanding some embodiments of the presentinvention, reference is first made to the construction and operation ofa standard boost converter as illustrated in FIG. 1.

FIG. 1 shows simplified representations of a switching mode powerconverter 10 in boost configuration. High electron mobility transistorHEMT 12 is driven by driver 14 to operate load 16 from power supply 18across inductor 20. The HEMT 12 is connected to load 16 across capacitor24, and diode 26 prevents reverse voltages. Graph 28 is a simplifiedgraphical representation of off and on states for the differentoperating regimes of the transistor in a boost converter.

In principle, in the boost converter of FIG. 1, each phase may have itsown converter and high electron mobility transistor HEMT 12 may beprovided as two GaN transistor devices in series, an upper device and alower device. In such a case, the lower device would act as a controltransistor and its voltage never goes above the gate voltage asguaranteed by the voltage of the upper transistor.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

Reference is now made to FIG. 2, which is a simplified block diagramshowing a circuit according to the present embodiments. An input 30applies a signal via a sequence of ring oscillators 32.1 . . . 32.n to apower output 32 to provide an amplified version of the input. The ringoscillator stages are thus connected between the input and the poweroutput, and as will be explained in greater detail below, each of thering oscillator stages comprises at least one enhancement modetransistor.

Reference is now made to FIG. 3, which is an exemplary circuit diagram40 corresponding to the block diagram of FIG. 2. In FIG. 3, a signalinput V1 42 feeds three ring oscillators 44, 46 and 48. Each ringoscillator has an upper enhancement mode 52 transistor and a lowerenhancement mode transistor 54. Power input 50 drives the gates of theupper enhancement mode transistors 52, and the outputs of the ringoscillators—in between the upper and lower enhancement mode transistors,drive the gates of the lower enhancement mode transistors 56 of thefollowing stages.

The enhancement mode transistor may for example be a SiC transistor or aGallium Nitride transistor, or more generally a wide bandgap transistor.

As shown in FIG. 3, a drive connection of any given stage is connectedvia an impedance 58, 60, to a gate of the following stage.

The ring oscillator stages may include a pull-up transistor and apull-down transistor respectively, and the drive connection that isconnected to the gate is between the pull-up transistor and thepull-down transistor. The impedance 58, 60, may be one or moreinductors.

As shown in FIG. 3, the drive connection 64 of a final oscillator stage48 is connected via an impedance 66 to the gate of transistor 54 of thefirst ring oscillator stage 44.

Each ring oscillator stage 44, 46, 48 is connected via an inputimpedance 68, 70, 72, to the input 42. The input impedances may beinductances as illustrated.

As shown in FIG. 3, a cascade of three of the ring oscillator stages maybe used. In other embodiments, there may be five or seven such stages.

Thus, as shown in FIGS. 1 and 3, a power converter may be constructedfrom an input, a power output and ring oscillator stages connecting theinput to the output. A drive connection of a first stage is connectedvia an impedance to a following stage, as discussed.

Likewise, as well as a power supply circuit, an amplifier may beconstructed in the same way, to comprise an input, a power output and aplurality of ring oscillator stages connected between the input and thepower output. As before, the ring oscillator stages each include oneenhancement mode transistor and may include two such transistors in apull-up, pull-down pair.

The embodiments may further comprise a switching circuit having a mainswitch circuit having a gate as a control input. A ring oscillator isconnected as a driver circuit to the gate to drive the main switch viathe gate.

In more detail, FIG. 3 illustrates a power converter according to thepresent embodiments, which is based on a ring oscillator circuit usingGaN HEMT devices in a cascade configuration as linear drivers forming a3-stage ring oscillator. The current in each leg flows only when bothupper (load) and lower (drive) transistors are high. The uppertransistors are all connected to the same voltage as the upper limit forthe gate-to-source voltages for both upper and lower transistors. Thevoltage on the upper transistors is limited by the applied voltage plusthe maximum drop across the top (load) inductors.

The principle of the circuit of FIG. 3 is that there is no control andall three phases are guaranteed to be 120° out of phase, The current ineach phase is controlled by the voltage supply, V1-42. The resultingaverage voltage across the upper drain device is V1, while the currentcharges the inductors. The output current is taken from the secondaryleads of each of the inductors indicated. A sample output showing thegate voltages of the lower transistor along with the drain voltage ofthe upper transistor is shown in FIG. 4.

More particularly, FIG. 4 shows a SPICE simulation of one phase showinglower gate voltage 200 and upper Drain voltage 210. The Ring Oscillator(RO) platform of power converter delivers continuous power without anyneed for external frequency modulation or circuits. The result is anefficient boost converter operating at high enough frequency (16 MHz orgreater) to minimize the needed inductors. A variation of the design ofthe present embodiments is shown in FIG. 5 in which a DC-DC converter 70sends output from the drain to an output capacitor C2 72. Again threering oscillators are shown 74, 76, and 78, although other numbers ofoscillators may be used, in particular but not exclusively odd numberssuch as 5 and 7. The ring oscillators are connected together as before,so that the oscillator drives the gate of the lower transistor of thefollowing oscillator and the final oscillator is connected back to thefirst. An inductance of 10 nH is easily achieved by inserting a 1 cmtrace in the circuit board. Such a self-oscillating device is based on aRO circuit using all GaN devices, having extremely low on-stateresistance and very low gate capacitance, allowing for much higherfrequency conversion in a self-regulating RO circuit.

The resulting voltage and current waveforms have been simulated and areseen in FIG. 6 which shows that the voltage increases with eachoscillation until the output power is met by the limitations of theboost transistors.

Reference is now made to FIG. 7, which is a simplified diagram showing afour transistor embodiment. It is known in the field that a rectifiercan be replaced with a transistor to provide a Synchronous Rectifier.Simply, the synchronous rectifier is another power switch that can becontrolled with isolated complimentary inputs. Since GaN devices arelimited to operating as enhancement-mode only devices with limitedallowed gate voltages, a specialized converter can be made with a basicring oscillator acting as a rectifying switch connected to ground andthe upper transistor of the ring oscillator can be connected to the highvoltage. In circuit 80, a driver 82 is fully isolated by fourtransistors 92, 94, 96 and 98. The four transistors are of differentsizes and hence of different capacitances, and numbered in order fromlargest to smallest the transistors viz: 92, 94, 96 and 98. The fourtransistors are connected in series to make oscillator converter. Thecircuit may be stacked in place of a rectifying switch since it isisolated.

Any number of tiles of the illustrated circuit maybe connected in seriesto get the appropriate power, although odd numbers such as 3, 5, 7 etc.are preferred.

An advantage of the circuit is that the only current drawn is to chargethe main switch. Thus the power dissipated=Qgs×Vg×F and is thusproportional to frequency, giving for example at 50 MHz, 20 nC*3V*50MHz˜3 Watts per Stage.

The energy may be recovered, thus the loss in the switch is due tocharging of the main switch. The loss may be recovered by discharginginto the next gate. Hence, the use of inductances as pull up and pulldown drivers and to provide feedback to the next stage.

Just recently (2018), a high switching frequency GaN-based bidirectionalDC-DC converter was mentioned in the professional literature. It hasshown that the conduction loss increases accordingly when the dead timeis longer due to a larger forward voltage drop of GaN devices, thussetting up ideal switching conditions for GaN devices are more severethan their Si counterparts. The present embodiment may solve thisproblem in that three phase RO converters may inherently provide a moreeffective three phase conversion due to ideal precise timing within thering oscillator stages.

Reference is now made to FIG. 8, which is a simplified diagram showing abuck converter 140 made with ring oscillators 142, 144 and 146 accordingto embodiments of the present invention. It is well known in the fieldthat the rectifier can be replaced with a transistor as a SynchronousRectifier. Simply, the buck converter is another power switch that canbe controlled with isolated complimentary inputs.

Since GaN devices are limited to operating as enhancement-mode onlydevices with limited allowed gate voltages, a specialized converter canbe made with the basic ring oscillator having upper and lowertransistors, the lower device acting as the rectifying switch connectedto ground and the upper device can be connected to the high voltage, asshown in FIG. 8.

Reference is now made to FIG. 9, which is a simplified circuit diagramof a Boost converter made using ring oscillators according to anembodiment of the present invention. It is generally known in the fieldthat a Boost converter, as mentioned, one that converts a lower voltageto a higher voltage, may be configured by reversing the coil and diodefrom a buck converter. The principle of operation involves having aswitch short a supply voltage to ground through the inductor until thecurrent increases to a maximum value before the switch is turned off.The voltage then commutates to the output diode (or synchronous switch).Such a converter can also be connected through a ring oscillator asshown in FIG. 9, where the gate of a main switch 100 is driven via asequence of three ring oscillators 102, 104 and 106.

In FIG. 9, the main Switch 100 is connected to the output of the thirdRO circuit 106 at the gate. The gate is connected to the common nodeconnection from stage 106, which RO stage is in turn connected to thecommon node connection of the preceding stage 104, and that to thecommon node connection of the first RO stage 102. Connected to the mainswitch 100 is main inductor 108. The main inductor 108 is the circuitelement that transfers current from the supply voltage 110 to the outputcapacitor 112 and the output network 114. The output can be anyelectrical means of withdrawing the electrical energy from the outputcapacitor 112. In this example, the load is a current source 116.Frequency is determined by the gate resistors 120, 122, 124, which arecombined with the input capacitance of the RO transistors (the gatecapacitance). Two or more options can lead to control of the outputvoltage. The fixed gate voltage (Vg) selects the maximum gate-to-sourcevoltage on all the transistors, which is a feature required by today'sGaN power devices. The resulting output of the RO circuit drives theoperation of the converter by modulating the oscillator voltage (Vosc).The voltage is chosen with respect to the threshold voltage of the mainswitch relative to the output of the common node from the RO circuit.Each circuit element is repeated N times based on the number of stagesof the RO circuit. A prototype according to FIG. 8 was able to work at afrequency of 50 MHz.

The output voltage from the RO cascade in FIG. 9 is shown in FIG. 10.Reference numeral 130 indicates the voltage seen at the gate-input tothe main switch. In this example, Vosc=1V and Vg=7V. The result is atriangle wave, typical of a Pulse-Width Modulation (PWM) input of astandard buck, boost or buck-boost converter. In today's technology,there is a very strict requirement that the gate to source voltage neverexceed 6 volts. The upper, load transistor guarantees this bymaintaining a maximum voltage Vg, above which the common node may notexceed. Where there is an offset voltage applied at Vosc, the maximumvoltage would be 6+Vosc, so 7V is the applied voltage at the gate node.FIG. 10 illustrates a voltage waveform 130 typical of the RO circuitmeasured at the common node and 132 the gate-to-source voltage for theload transistor of the RO circuit.

Reference is now made to FIG. 11 which is a circuit diagram of a D-typeamplifier using ring oscillators according to embodiments of the presentinvention.

The D-type amplifier 130 sends output from the drain to an output 132.Again three ring oscillators are shown 134, 1366, and 138, althoughother numbers of oscillators may be used, in particular but notexclusively odd numbers such as 5 and 7. The ring oscillators areconnected together as before, so that the oscillator drives the gate ofthe lower transistor of the following oscillator and the finaloscillator is connected back to the first.

The resulting DC to DC converter puts out a voltage signal that isdirectly proportional to the control voltage at the gate (V2) in theillustrated circuit. Hence, if the input signal is slow compared to theoscillation frequency, then the filtered output voltage will be a directamplified image of the input voltage. In an example, the input voltageof 2.5V gives an output of over 12V, but that is driving a load of 1000Ohms. Considering the input is only the gate voltage of the Loadtransistors (as shown) then the input power may be as low as 1 W with anoutput power as high as 100 W, giving very high power transfer gain.

The amplifier makes use of the ring oscillator circuit of the presentembodiments and may include GaN HEMT devices in a cascade configurationas linear drivers forming a 3-stage ring oscillator. The current in eachleg flows only when both upper (load) and lower (drive) transistors arein a “high” state. The upper transistors are all connected to the samevoltage as the upper limit for the gate-to-source voltages for bothupper and lower transistors. The voltage across the upper transistors islimited by the applied voltage plus the maximum drop across the top(load) inductors, indicated in the SPICE model.

The principle of this circuit is that there is no need for a precisepulse control and all the three phases are guaranteed to be 120° out ofphase. The current in each phase is controlled by the main voltagesupply, V1. The resulting average voltage across the upper drain deviceis V1, while the current charges the inductors. The output current istaken from the secondary leads of each of the inductors. The circuit mayoperate at over 100 MHz and prototypes have demonstrated GaN RO circuitsup to 650 MHz. The output control occurs via simply providing feedbackto the gate voltage (V2) so that the output is at the desired level.This has been implemented both in buck and boost versions. Buck-Boostand other applications are in the plans with the ability to make theconverters as efficient as possible while maintaining very highfrequency oscillation.

A working prototype of the device used small coils and inductances canbe implemented as wire traces on the circuit board. The output voltageis across a 200 Ohm load showing about 2.5 to 1 conversion from theinput voltage of 3.6 Volts. The prototype had an output voltage across a200 Ohm load, for a main input voltage of 3.6 V and a control gatevoltage of 2.3 V.

The oscillation waveform is seen in FIG. 12, showing all 3 phases of thegate voltage, that is the gate voltage for each of the threeoscillators. We see that the peak-to-peak voltage is 2.44 Volts,demonstrating excellent control on the lower voltage device. Thisteaches that the lower devices can be much lower capacity with highcurrent while the upper devices can be very high voltage GaN devices.The circuit is operating at about 7.8 MHz allowing implementation withvery small components.

Reference is now made to FIGS. 13A and 13B, which are simplifieddiagrams illustrating how a circuit according to the present embodimentsmay be implemented on a single integrated circuit. FIG. 13A shows theunderside of an integrated circuit 150 with pins 152 for electricalconnections, not all of the pins being used in all cases. In order toconstruct a gate driver circuit 154 according to the presentembodiments, a single additional pin 156 is needed to connect the commonnode to the next phase, account being taken of the need to reduce totalgate resistance.

A prototype was made using the EPC2031 GaN enhancement mode powertransistor.

The following features were noted:

-   -   C(input) 1640 pF 2000 pF max    -   (Rgate+Rdriver (pull-down))<250 mW    -   Achieved using Rgate=200 mW.    -   Rdriver kept to less than 50 mW (EPC2040 as the driver).    -   Pull-Down time ˜1 ns.    -   Pull-Up time is made longer to allow control of Duty Cycle.    -   With EPC2037 (0.5 W Rds,ON) Pull-Up time ˜3 ns.    -   Saturation time is ˜1 ns*2 (high and low)    -   With 3 phases per ring chain, Fmax=1 GHz/[3*(4 ns+2 ns)]=55 MHz.

Reference is now made to FIG. 14, which is a simplified circuit diagramof a switching circuit 160, again having three ring oscillators 162, 164and 166. Two GaN transistors in each ring oscillator 168, 170, act as apull up network and operate as a single PMOS. The correspondingpull-down network includes a single GaN transistor 172 that acts as asingle NMOS. A connecting inductance between each ring oscillator, 174,acts as a filter and allows the drivers to be separated on differentchips and connected together at package level.

Reference is now made to FIG. 15, which is a simplified diagram of afive stage switching circuit. In this circuit, working at 120 Watts perphase, pull down time is short compared to pull up time. Moreparticularly the on time is three times as long as the off time, givinga 3:1 duty cycle. The duty cycle can be controlled by increasing thepull-up resistance. However if heading above 3:1, the efficiency goesdown, as energy gets lost in the capacitors and the physical limits ofthe device are reached.

FIG. 16 is a similar five stage circuit and may be built as a singleintegrated circuit. Again the circuit has five ring oscillator stages,and a driver and pulse width modulation PWM together on the chip.Compared to the circuit of FIG. 15, this circuit has the same voltagebut higher current and thus a greater energy density. As a result, itrequires a slightly bigger inductor.

It is expected that during the life of a patent maturing from thisapplication many relevant enhancement mode transistors, GaN transistors,ring oscillators, wide bandgap transistors etc. will be developed andthe scopes of the corresponding terms are intended to include all suchnew technologies a priori.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment, and the text is to beconstrued as if such a single embodiment is explicitly written out indetail. Conversely, various features of the invention, which are, forbrevity, described in the context of a single embodiment, may also beprovided separately or in any suitable subcombination or as suitable inany other described embodiment of the invention, and the text is to beconstrued as if such separate embodiments or subcombinations areexplicitly set forth herein in detail.

Certain features described in the context of various embodiments are notto be considered essential features of those embodiments, unless theembodiment is inoperative without those elements.

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting. In addition, any priority document(s) of this applicationis/are hereby incorporated herein by reference in its/their entirety.

1. A power converter comprising an input, a power output and a pluralityof ring oscillator stages connected between said input and said poweroutput, the ring oscillator stages each comprising at least oneenhancement mode transistor.
 2. The power converter of claim 1, whereinsaid ring oscillator stages respectively comprise a second enhancementmode transistor.
 3. The power converter of claim 1, wherein the at leastone enhancement mode transistor is at least one member of the groupconsisting of a SiC transistor and a Gallium Nitride transistor.
 4. Thepower converter of claim 1, wherein the enhancement mode transistor is awide bandgap transistor.
 5. The power converter of claim 1, wherein adrive connection of a first stage is connected via an impedance to afollowing stage.
 6. The power converter of claim 5, wherein at least oneof said ring oscillator stages comprises a pull-up transistor and apull-down transistor.
 7. The power converter of claim 6, wherein saiddrive connection is between said pull-up transistor and said pull-downtransistor.
 8. The power converter of claim 5, wherein said impedancecomprises an inductor.
 9. The power converter of claim 5, wherein adrive connection of a final oscillator stage is connected via animpedance to a first ring oscillator stage.
 10. The power converter ofclaim 1, wherein each ring oscillator stage is connected via an inputimpedance to said input.
 11. The power converter of claim 10, whereinsaid input impedance comprises an inductance.
 12. The power converteraccording to claim 1, comprising a cascade of at least three of saidring oscillator stages.
 13. The power converter of claim 1, wherein adrive connection of a first stage is connected via an impedance to afollowing stage.
 14. (canceled)
 15. The power converter of claim 13,wherein said ring oscillators respectively comprise at least twoenhancement mode transistors.
 16. (canceled)
 17. The power converter ofclaim 13, comprising three of said ring oscillators, each of said threering oscillators driving a switch for a different phase of a three phasesystem.
 18. An amplifier comprising an input, a power output and aplurality of ring oscillator stages connected between said input andsaid power output, the ring oscillator stages each comprising at leastone enhancement mode transistor.
 19. The amplifier of claim 18, whereina drive connection of a first ring oscillator stage is connected via animpedance to a following ring oscillator stage.
 20. The amplifier ofclaim 19, wherein a drive connection of a final oscillator stage isconnected via an impedance to a first ring oscillator stage.
 21. Theamplifier of claim 18, being a D-type switching amplifier.
 22. Aswitching circuit comprising: A main switch circuit having a gate as acontrol input; and A ring oscillator connected as a driver circuit tosaid gate to drive said main switch via said gate.